Human Intestinal Parasites – Symptoms and Treatment

What Are Intestinal Parasites?

Human intestinal parasites are either one-cell organisms or intestinal worms that live in the small or large intestine and use the stool or blood from intestinal wall as a source of food.

One-Cell Parasites – Protozoa

One-cell organisms, like Giardia lamblia, also called Giardia duodenale (Picture 1), Cryptosporidium (crypto) and Cyclospora, utilize nutrients from the stool. They belong to a living kingdom Protozoa (Gk. protos = first; zoa = animals). They may cause inflammation of the small intestine thus hampering absorption of nutrients. Entamoeba hystolytica lives predominantly in the colon.

Giardia under electron microscope

Picture 1. Parasite giardia from a stool sample,
as seen under the electron microscope (actual size:~0.01 mm)

Intestinal Worms (Helminths)

Intestinal worms (helminths), like roundworms (hookworms, whipworms, Ascaris and Trichinella), tapeworms and flukes, are few millimeters to several meters in size, they eat the bowel content or suck the blood from the intestinal wall and can cause about the same symptoms as one-cell parasites.

How Can One Contract Parasites?

Parasites are shed in human or animal stool; a source of an infection in poor countries is usually stool-contaminated food or water, and in industrialized countries, recreational water (lakes, pools) or homosexual men (1).

Symptoms of Intestinal Parasites

Intestinal parasites can cause one or more of the following symptoms:

  • Mild diarrhea with waxing and waning course, appearing few days to several weeks after the travel into the country with poor hygiene, and lasting from several days to months (for comparison: bacterial diarrhea usually has a sudden and dramatic onset within some hours after a meal, and heals on its own in few days).
  • Mucus in the bowel movement
  • Bloating and flatulence
  • Weight loss is common.
  • Pale skin, tiredness, tingling (due to reduced absorption of minerals and vitamins).
  • Entamoeba histolytica may cause severe colitis with ulcers, abdominal cramps, blood in the stool with occasional pus – the disease is known as amebic dysentery (9). Entamoeba may invade liver, lung, brain, or other organs, where it forms cysts.
  • Worms, their parts or eggs can be sometimes found in the stool. Read more about intestinal worms.
  • Infection by either one-cell organisms or intestinal worms is often present without any symptoms.

Diagnosis of Intestinal Parasites

Diagnosis can be often suspected from history of prolonged bloating or diarrhea and can be confirmed by the following tests:

  • Ova and parasites (O&P) test of the stool. The test is often false negative so three stools from three different days (at least two days apart) should be tested.
  • Hemoccult test may reveal blood in the stool
  • Blood tests often reveal elevated eosinophils and IgE antibodies and lowered ferritin, hemoglobin or red cells
  • CT or biopsy are needed to find cysts in the liver, lungs or brain.

Treatment of Intestinal Parasites

  • Anti-parasitic drugs, like metronidazole, quinacrine, tinidazole and furazolidone are usually effective, but the exact treatment regime, as prescribed by your doctor, should be followed. Paramomycin is not absorbed from the intestine into the blood, so it may be used in pregnancy (1). For cyclospora trimethoprim-sulfametoxazol can be used (2).

Intestinal parasites are usually successfully treated, but may reoccur if the source of infection still exists.

Prevention of a Parasitic Infection

An infection by intestinal parasites can be prevented by:

  1. Hand washing with soap before eating and after bowel movement
  2. Food cooking kills all parasites (but smoking or freezing does not)
  3. Washing of raw vegetables and fruits
  4. Travelers from western countries visiting Central or South America, Africa or South Asia should ingest only packaged, bottled or cooked foods and drinks and only fruits that they can peel by themselves. Read about food disinfection in the article about travelers diarrhea.

For How Long Is a Person with Parasitic Diarrhea Contagious?

The stool of a person with intestinal parasites is contagious as long as infection lasts, or, like in infection with Entamoeba hystolitica, possibly for years after symptoms cessation. The stool of an infected person without any symptoms is also contagious.

Related Articles:


  1. Incidence of intestinal parasites in homosexual men (
  2. Intestinal protozoa (
About Jan Modric (249 Articles)
Health writer

Please note that any information or feedback on this website is not intended to replace a consultation with a health care professional and will not constitute a medical diagnosis. By using this website and the comment service you agree to abide by the comment terms and conditions as outlined on this page

  • Jan Modric


    are “white things” in your stool bright white, or like a whitish/translucent mucus?

  • Jan Modric


    sugar craving and increased appetite in general and the resulting increase of body weight plus finding a worm in the stool all speak for – intestinal worms. Black dots could be related to worms, but I can’t exclude other causes, like clumps of bacteria, blood or food particles. For you it is not very important to know which exact parasite you have, since most prescribed anti-parasitic drugs work against most parasites. Levamisol is a bit unusual choise, though. They are only prescribed antiparasitic drugs that are currently considered as effective and safe. If your symptoms lessen considerably in the next days, and disappear completely with time, it means the treatment was effective.

  • just wondering


    I was wondering, the anti-parasitic drugs that doctors prescribe sound effective, but you have mentioned the possibility of intestinal worms invading other areas of the body, such as the lungs, liver, heart and kidneys. Will anti-parasitic (prescribed) drugs kill only parasites in the intestine, or will they also get rid of infections in other parts of the body? If only the intestines, it seems that re-infection would be quite likely.

  • Jan Modric

    just wondering,

    in a severe infection, or in people with lowered immunity, parasites may enter through the intestinal wall into the blood and from there into other organs, where they can form cysts. It depends on the type of the parasite, but a bowel re-infection with parasites originating from the cyst is not very likely. It is usually the cyst itself that cause more problems – they can be detected by a CT or MRI, and often blood eosinophils or/and IgE antibodies are elevated. Inappropriate antiparasitic drug or its dose, incomplete course of treatment, or another infection are more likely causes of re-infection.

  • Houtte

    I have a question, for a few months on and off I suffer from gas and sulfur burps with loose stool, it is never consistent, it can last for a day or two and never come back for months, what could this be?

  • Jan Modric


    here is a list of foods high in sulfur and list of possible causes of sulfur burps. I’m not sre if intestinal parasites could cause sulfur burps lasting for only a couple of days. Do you also have sulfur smelling gas? Are you completely symptoms-free between the sulfur burps episodes?

  • 4 weeks

    Hi Jan,

    Just writing to ask your advise on what i think to be some form of parasites in my stomach. I am currently travelling in south east asia and have had the same symptoms every day for the last 4 weeks. It began when I was in Laos so of course was exposed to many unsanitay foods and ate from street stalls every day.
    Every time I eat (any type of food) i pass the same looking stool within 5 to 15 minutes after my first bite. The stool is bright yellow liquid with small mushy peices of solids small in size like a pea. When this began they were black in the yellow liquid now they are yellow like the liquid.
    Right now i dont have access to a doctor who can do your above suggested forms of testing for parasites.
    A friend of mine who was a woofer in the marines has suggested I take a 5 day course of a drug that is designed to kill all 4 types of parasites.
    Do you know what drug this is and do you think this is a good idea?
    Thanks for your advise in advance :)

  • Jan Modric

    4 weeks,

    no I don’t know the name of such a drug. In general non-prescribed antiparasitic drugs are less effective and have more side effects than prescribed ones. Many prescribed antiparasitic drugs act against several types of parasites and maybe some doctor would be willing to give you such a medication without running a test before that. I can’t take responsibility to recommend any specific medication, though.

  • never treated

    Aprox 23 yrs ago I was diagnosed with giardia parasite. However, I was preganant and was not treated. After I had my son, the symptoms went away. I am curious if it could be he cause of my medical issues Im currently experiencing.

    I have seen neuro doc for over a yr for unexplained left side face pain (like sinus pain), droopy eye, dialated pupil, facial nerve damage,eye pain and pressure, loss of vision in left eye and high blood pressure. (medical mystery). They are leaning toward MS or Tolosa Hunt..but still no deffinate diagnosis. All MRI’s have been clean with no leisions on the brain.

    so, I am just courious if by any chance the parasite I many years ago could be causing any of these issues.

    Thank you,

  • Jan Modric

    never treated,

    giardia does not sound like a possible cause of your symptoms to me. To confirm/exclude MS, a lumbar puncture and investigation of the cerebrospinal fluid may be needed. At the same time, tests for Borrelia (Lyme disease) may be performed.

  • confused

    Hi, I went to see an allegry specialist today and he said i may have parasites? I don’t know what the symptoms are however I told him I was breaking out in hives and he said that i have seasonal allergies however he thinks i may have parasites. Is this a symptom????

  • nmorgan

    RE: “never treated”: ABC world news just did a story on a woman with similar symptoms (one droopy eye, etc.). She had her metal fillings removed and gold put in her teeth. She recovered. (see ABCwebsite, maybe)

  • Jan Modric


    intestinal parasites may cause an allergy-like rash (hives); a stool test is needed for diagnosis. If you live on the northern hemisphere than a late December is not exactly the season for seasonal allergies. Hives may also occur in food allergies (few minutes to several hours after taking the problematic food), allergies to certain substances, and severe psychical stress.

  • Tims

    Hi, I’ve been having this recurrent problem – light yellow semi-liquid stool with hard pieces in it, urges to defecate without result, gases, etc.
    I tried to change my food habits – eating mostly rice, boiled chicken and vegies – doesn’t help one bit.
    I also started to have this feeling of having something stuck in my throat, and the weirdest thing – I feel nausea when I take deep breaths
    Can it be worms or something?
    What would you recommend to do?

  • 8675309

    Hi, for the last month ive been having a very intense anal itch. Im getting plenty of fiber now, no food allergens i use mild soaps and detergents, eliminated spicy foods. There are no visible hemmroids or fissures. Just to be safe i treated myself for worms, hemmroids, fungal and yeast infections all with no luck. The only thing that seems to help is ibuprofen but the itc h is still there. I hope you can help!

  • Jan Modric


    there are not sufficient data in your question to give you any opinion. Apart from intestinal parasites, possible causes include food intolerances, like fructose malabsorption, celiac disease, lactose intolerance, food allergy, or an inflammation of thee bowel (Crohn’s disease, for example). I recommend you to check this personal medical history questionnaire, write down all relevant details and then visit a gastroenterologist.

  • Jan Modric


    intestinal parasites, particularly pinworms, would be possible, but there are several other causes of anal itch.

  • marcpogi

    what is the symthoms of human intestinal parasite…how can i detect it?

  • Jan Modric


    symptoms of intestinal parasites are described above in the article and may include diarrhea, bloating, skin rash and other symptoms, but it is important to know that sometimes there are no symptoms at all. Parasites are detected with a stool test for parasites.

  • sletchh

    Hi I just came back from Mexico and I tried really hard not to drink the water but I did have mixed drinks with ice, so that may have to do with this. My stomach has been bothering me for the past four days. It`s gotten a little better but it doesn`t go away. I drank a lot of alcohol the night before it started hurting so I thought maybe it was that, but I don`t think it would have lasted for three days. Could it be the food? I`m nervous it is a parasite. What do you think?

  • pilgrim

    Was wondering if you could help me? I have hot flash type episodes, constipation & back pain. tonight I took a bath and when the hot water passed over my belly I watched my belly move. I can feel the movement(sorta like when I was pregnant). The last time I was hospitilized for 8 days they told me that the ct-scan showed a swollen colon and then they told me that it must be a “bad reading” Could it be a parasite? thanks for answering.

  • halucinated

    Hello, one morning i had a stomachache and i went to the toilet fast. the poop is just water and suddenly i saw a beetle like insect in the bowl. it couldn’t have flown in there because i was looking at the poop coming out. It’s a habit. The size is like a half inch long.
    I told my doctor about it and he says I’m hallucinating.
    Could it really coming from intestines?
    What should i do?
    I had other symptoms like itchiness and bleeding after poop.
    Please advice

  • Jan Modric


    coming from Mexico is a good reason to think about food poisoning. Bacterial food poisoning usually results in obvious diarrhea which usually resolves within 5 days. Intestinal parasites may trigger less obvious abdominal problems, like cramps, bloating, mild diarrhea, but symptoms usually last longer – if not treated, for weeks or even months. I’m not aware of any remedy for intestinal parasites, so if you do not get better in few days, it could be good to ask a gastroenterologist for a stool test.

  • Jan Modric


    I can’t exclude parasites, but there are other causes of “swollen colon”, like a Crohn’s disease or ischemic colitis. Hot flashes commonly occur in carcinoid syndrome. Hard to say more from the information you’ve provided, so I recommend you to discus with a gastroenterologist about that “bad reading” and eventual other investigations, such as colonoscopy.

  • Jan Modric


    you can visit a gastroenterologist, who can decide which tests to perform to find out the cause of the bleeding and itchiness. The doctor can also order a stool test for parasites.

  • wormed

    Spring of 2009 I ate sushi and about 6 months later fished an asian tape worm out of the toilet (16 ” long) I scooped it up and drove to the emergency room. At the time I had dropped over 25lbs(made me 103lb’s & a childrens sz 10 – really light for an athlete) and I never really had any symptoms besides the weight loss. I didn’t think anything of it in the begining, because I was training 2x’s per day. I did however have major stomach problems the night before however, I thought it was something I ate at a birthday party (mainly because I am allergic to all milk products) The worm had the formed head attached so the ER told me not to worry about re-growth. They did not however give me any medications. It has now been almost 2 years and I did gain the weight back quickly (3 months) and maybe a few lb.s more, My concern now is I have had extreme bloating with everything I eat or drink, including water sometimes and my stomach bloats to the point I can’t see my feet. I am normally a size 0, so not seeing my own feet while standing, is quite strange for me. It can be quite painfull, sometimes the bloat is still there in the morning. I have also become constipated over the last 4 months, so I take Dulcolax on a regular basis. However there are days I have no problems, as long as I don’t eat or drink anything, and my stomach is back to flat as a pancake. I have a physical scheduled with my Dr. and I want to see if there are any test/treatment in particular that I should ask for.

  • Dr. Chris

    Hi Wormed

    This could be due to a disturbance in your normal intestinal flora (normal bowel bacteria) and you will need to take a probiotic. Please speak to you doctor about specific probiotics, preferably those containing Saccharomyces boulardii and/or Lactobacillus casei. Many cheaper and “all natural” probiotics lack these microorganisms. Your pharmacist will be able to advise you further. The other possibility here is that it could be irritable bowel syndrome (inflammatory bowel a possibility but more symptoms should be present). Excessive bloating for no known reason (no excessive gas) after eating is a typical symptoms of IBS. Ideally you should see a gastroenterologist who could consider further investigations, like a colonoscopy, and possibly run a full stool analysis to identify any underlying cause. It is possible that this is unrelated to your previous parasitic infection so rather let a medical professional examine you objectively.

  • Worried

    Hi, I’ve been experiencing constipation on and off since Dec. and have taken miralax about 4 times to help the symptoms when they come. About 1 month ago I was constipated and had about 4 to 5 bowl movements, and the stool was dark almost black and each stool had one strand like thing coming out of it. Even some strands just by themselves. I’m pretty sure I took some miralax that day and then the constipation went away and I didn’t see the strands anymore, until this morning. My stool was big and light in color and I had the strands coming out of it again. They almost look like tampon strings. They arn’t white in color they match the color of my stool both times. They kind of look like roots just slighter bigger then a root. I’m worried that it might be a parasite, but I haven’t had any of the typical symptoms that come with parasites. I’ve also inspected the strings to see if they were moving and they are not.

  • Earthen

    Very informative site. My family and I frequent the caribean islands between the Bahamas and the us virgin islands. Recently within the last 2 years I started a host of digestive problems including terrible insomnia/anxiety. I eat a very organic natural diet. Since our last visit to the islands (eating the local fruit from a fruit stand) I have came down with major stomach pain in the upper right ( under rib cage) very tender to touch, with constant burping ( that seems worse if I haven’t eaten but is always there). I was seeing a natropath and he had me on whey protein, that is when things got really bad and I became allergic to dairy. I started to avoid wheat, grains, dairy, etc symptoms decreased slightly but were unbarable if I ate sugar. Wheat or dairy. The pain has now speed to my neck (full feeling in the throat, swollen lymph node on right side). I did herbal anti parasitics but I can’t take much wormwood etc as my left side ( low rib cage area starts to burn). I have constipation problems. I read that switching to raw goat milk can help with Intestinal issues so my family started on the raw goat, recently my 2 yr old has been very sick, very constipated, dr says he’s boarder anemic. He can not poop even with prune juice etc, I have to give him suppositories. He cries that his belly hurts, dr prescribed him medebanzole and he is more constipated and sick than before. Could worms be stuck in him? As for me I have seen long clear jelly like things after giving myself enema, including stool encased in a clear jelly like tubing-creepy! I have no idea what is going on.

  • Worried Mom

    While bathing my daughter the other night I noticed something coming out of her anus. I thought at first that it was a piece of dead grass, but it was definitely some kind of flat brownish worm. I called the pharmacy and they gave me combantrin which is for round worm and pin worms. I have looked up pictures for both of those, but they do not look like what I pulled out of her. could it be something else? Should I take her to the doctor or am I over reacting?

  • Dr. Chris

    Hi Worried

    Sometimes the typical symptoms associated with intestinal parasites is not present. This also depends on the type of parasite. However there may be other factors to consider like fiber which cannot be digested and even mucus. Speak to your doctor about your symptoms. A stool sample would help to confirm or exclude the presence of any parasites – even if the parasite itself is not in the stool, ova (microscopic) may be detected during laboratory investigation. Other tests may also be useful in diagnosing the parasites.

    Apart from the possibility of parasites, your constipation associated with this black stool is a bit of a concern. There are many possible causes, some of which may be very serious in nature. You should at this point see a gastroenterologist and possibly have a colonoscopy if he/she deems it necessary.

  • Dr. Chris

    Hi Earthen

    In terms of your son, it is possible that it is worms and your doctor can conduct a stool test to verify this. In your case, it is obvious that you have a preference for complementary therapies but do not turn your back on proper medical investigation, especially since all these alternative measures have not yielded the desired results. I assume that you do know the dangers of using large amounts of wormwood herb. A stool test would also be advisable for you to verify whether it is intestinal parasites or possibly other conditions like those affecting the liver, gallbladder or even pancreas which could account for the appearance of your stools recently. Speak to your doctor and see a gastroenterologist if possible.

  • Dr. Chris

    Hi Worried Mom

    Yes, please take your daughter to the doctor immediately. If you have kept this brownish “worm” then it may be helpful to take it through to your doctor. Alternatively a stool test may be conducted. It is advisable not to self-medicate in this manner, especially with children. While your pharmacist has good intentions and is there to help you, this service should not replace a consultation with a doctor, especially when it comes to children.

  • Terry

    Dr Chris,i have had soft stools + occasional diarrhea now for 3 months off an on. Usually no more the twice daily, sometimes accompanied by a feeling of malaise It stated after I had a severe cold. I was already experiencing the diarrhea symptoms for a few days and my general Dr prescribed azithromycin for 6 days for the cold. I also took clarithomysin for campylobact 3 weeks later..which the test said I had then said I didn’t! Have had 2 stool tests for parasites and a colonscopy..all negative. My gastro Dr says it’s likely the bacterial r compromised and I should improve over time. I’m taking probotics….but my condition remains the same. Any thoughts on what I should do?

  • Linda

    I was diagnosed 8 weeks ago with a parasite, giardia lamblia, in my stomach. I took 3 different kinds of antibiotics. Seemed much better. For a week I felt the best I had in months. However, yesterday I started getting nauseated and now have frequent stools again that look like coffee grounds. Do you think this parasite has come back? I don’t know where I picked this us at. I had my water tested but it was ok. I also have diverticulosis. Should I call my doctor? I was so happy to feel good again. It just didn’t last very long. Help.

  • Eddie


    For the last couple months I’ve been experience itching anus. I’ve been to the doctors many times but there not sure. My stools lately have been pebble like with it looks kinda of like small eggs or corn in my feces, but I’m not eating corn. I sometimes feel bloated and I never seen blood in the toilet, only sometimes on the toilet paper, very little. Can you help me please, does it sound like a parasite.

  • Dr. Chris

    Hi Terry

    Follow your gastroenterologist’s advice as he knows your case more thoroughly. Hopefully you are using the probiotics he prescribed which has Saccharomyces boulardii and even Lactobacillus casei. Not all probiotics are the same so don’t buy anything over-the-counter at your local supermarket. It takes a few days to about a week or so. But if your symptoms are not improving then go back to the doctor. Otherwise wait and see.

  • Dr. Chris

    Hi Linda

    Yes, you should see your doctor again so that you can be reassessed as there are many other causes related to your giardia infection that may account for the bowel movements. The infection could have recurred but it is also important to note that lactose intolerance may develop with giardiasis. This could also account for frequent bowel movements or diarrhea. Antibiotic associated diarrhea (AAD) is another consideration here. The coffee ground appearance of your stools may indicate old/dark blood in the stool (melena). This may be an indication of upper gastrointestinal bleeding which needs to be investigated as well. Cannot definitely say that this is related to the giardiasis, antibiotics, diverticulitis, complications or side effects. Your doctor will be able to help you further.

  • Dr. Chris

    Hi Eddie

    This could be constipation which may be bruising the anus and even contributing to the development of a hemorrhoid hence the bleeding upon wiping. Take a look here at the different types of feces. The particles that you may be seeing are just bits of undigested food – it is not always easy to corroborate with what you ate due to the degradation that occurs in the gut. It is unlikely parasite eggs (ova) because these would not be so visible. This is just possibility though. Your doctor should consider a stool test at this point to identify the presence of any ova, digestion or absorption problems and so on. See a proctologist or even follow up with a gastroenterologist if necessary.